Very small matter for a piper's son (4)
I believe the answer is:
atom
'very small matter' is the definition.
(I've seen this in another clue)
'a piper's son' is the wordplay.
'piper's son' becomes 'tom' (I've seen this in another clue).
'a'+'tom'='ATOM'
'for' is the link.
(Other definitions for atom that I've seen before include "A small but mighty particle" , "Minute fragment" , "Piece of matter in moat" , "Tiny bit" , "Smallest particle in an element able to take part in a chemical reaction" .)